The present invention concerns integrated circuits and, more precisely, temperature compensated oscillators.
In the present description, an oscillator is defined as a circuit able to provide periodic signals having a predetermined frequency which is substantially constant.
One problem encountered in providing such a frequency lies in the fact that it varies as a function of the temperature with an intrinsic temperature coefficient (of the oscillator), which is detrimental when the oscillator is intended to be used as a time base.
A large number of temperature compensated oscillators intended to overcome this problem exist in the state of the art, in particular in U.S. Pat. Nos. 5,604,467 and 5,180,995.
FIGS. 1a and 1b of the present description show oscillators 1 and 10 such as those described in U.S. Pat. Nos. 5,604,467 and 5,180,995 respectively.
As is shown in FIG. 1a, oscillator 1 includes a reference source 3 and supply means 5. Reference source 3 is arranged to supply a current I to supply means 5, and is temperature compensated so that the intensity of current I is constant, independently of the temperature. Supply means 5 include a capacitor 6 charged via the injection of current I, and is arranged to compare the voltage present across the terminals of capacitor 6 to a reference voltage Vref, and to supply in response a periodic signal CLK.
One drawback of such an oscillator lies in the fact that it does not allow the influence of the temperature on supply means 5 to be compensated, the latter having an intrinsic temperature coefficient. As a result, the frequency of signal CLK can vary under the influence of the temperature, via the temperature coefficient of supply means 5.
As FIG. 1b shows, oscillator 10 includes supply means 14 provided with inverters 15a to 15g and a reference source 12 provided with a constant voltage generator 13 and two resistors R1 and R2, the latter being formed so as to have positive and negative temperature coefficients respectively. Thus, following a temperature increase, the signal frequency xcfx86o tends to decrease in accordance with the temperature characteristics of the oscillator itself and of resistor R1, and tends to increase the temperature characteristic of resistor R2.
One drawback of oscillator 10 lies in the fact that, in order to prevent it becoming locked on a parasitic frequency, it has to include prime number of inverters (7 here), which causes a considerable occupation of surface area.
One drawback of resistor R2 lies in the fact that it has to have a negative temperature coefficient, while the resistors manufactured in integrated circuits generally have positive temperature coefficients.
Another drawback of resistor R2 lies in the fact that it is formed in a zone of polycrystalline silicon which is a material with low resistivity. Thus, to have a high reference voltage, it is necessary either to form a high value resistor, which requires a considerable occupation of surface area, or provide a significant electric current to reference source 12 (typically of the order of xcexcA), which causes a high level of consumption.
An object of the present invention is to provide an oscillator able to compensate the influence of the temperature over all the oscillator components, so as to supply a periodic voltage at a predetermined frequency, independently of the temperature.
Another object of the present invention is to provide an oscillator which answers the constraints as regards space requirement and electric power consumption, which are usual in the integrated circuit industry.
Another object of the present invention is to provide an oscillator which can be manufactured via the standard integrated circuits manufacturing processes.
These objects, in addition to others, are achieved by the oscillator according to claim 1.
One advantage of the resistor of such an oscillator is that it gives the reference source an equal temperature coefficient to that of the supply means, so that the temperature has the same influence on that voltage and on that means. As a result, the supply of the periodic voltage at the desired frequency is independent of the temperature. In other words, the resistor thereby formed allows compensation of the frequency variations, under the influence of the temperature on the supply means.
Another advantage of the oscillator according to the present invention is that it is formed of known components, which can be made in a monolithic manner, which answers concerns as to space requirement and complexity.
These objects, features and advantages of the present invention, in addition to others, will appear more clearly upon reading the detailed description of a preferred embodiment of the invention, given solely by way of example, in relation to the annexed drawings.